Americium

Americium (pronEng|ˌæməˈrɪsiəm) is a synthetic element that has the symbol Am and atomic number 95. A radioactivemetallic element, americium is an actinide that was obtained in 1944 by bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for the Americas, by analogy with europium. Americium is widely used in commercial ionization-chamber smoke detectors.

Properties

Pure americium has a silvery and white lustre. At room temperatures it slowly tarnishes in dry air. It is more silvery than plutonium or neptunium and apparently more malleable than neptunium or uranium. Alpha emission from 241Am is approximately three times that of radium. Gram quantities of 241Am emit intense gamma rays which creates a serious exposure problem for anyone handling the element.

Americium is also fissile; the critical mass for an unreflected sphere of 241Am is approximately 60 kilograms. It is unlikely that Americium would be used as a weapons material, as its minimum critical mass is considerably larger than more readily obtained plutonium or uranium isotopes. [cite web | publisher = International Panel on Fissile Materials | title = Fissile Materials & Nuclear Weapons: Introduction | url = http://www.fissilematerials.org/ipfm/pages_us_en/fissile/fissile/fissile.php | accessdate = 2007-11-22]

Applications

Americium can be produced in kilogram amounts and has some uses, mostly involving 241Am since it is easiest to produce relatively pure samples of this isotope. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector contains a tiny amount (about 0.2 microgram) of 241Am as a source of ionizing radiation. This amount emits about 1 microcurie of nuclear radiation when new, with the amount declining slowly as the americium decays into neptunium, a different transuranic element, with a much longer half-life (about 2.14 million years). With its half-life of 432 years, the americium-241 in a smoke detector includes about 5% neptunium after 22 years, and about 10% after 43 years. After the 432-year americium-241 half-life, a smoke detector's original americium would, by definition, be more than half neptunium.

241Am has been used as a portable gamma ray source for use in radiography. The element has also been employed to gauge glass thickness to help create flat glass. 242Am is a neutron emitter and has found uses in neutron radiography as well as a neutron emitting radioactive source used in well logging applications (241AmBe). It has also been cited for use as an advanced nuclear rocket propulsion fuel. [cite news | title = Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks | publisher = ScienceDaily | date = 2001-01-03 | url = http://www.sciencedaily.com/releases/2001/01/010103073253.htm | accessdate = 2007-11-22] This isotope is, however, extremely expensive to produce in usable quantities.

History

Americium was first isolated by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in late 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as Argonne National Laboratory). The team created the isotope241Am by subjecting 239Pu to successive neutron capture reactions in a nuclear reactor. This created 240Pu and then 241Pu which in turn decayed into 241Am via beta decay. Seaborg was granted a patent for "Element 95 and Method of Producing Said Element," whose unusually terse claim number 1 reads simply, "Element 95." [US patent|3156523] The discovery of americium and curium was first announced informally on a children's quiz show in 1945. [cite web | title = It's Elemental: The Periodic Table: Americium | author = Rachel Sheremeta Pepling | publisher = Chemical & Engineering News | url = http://pubs.acs.org/cen/80th/print/americiumprint.html]

Isotopes

(231Am) to 249.078 u (249Am).

Chemistry

In aqueous systems the most common oxidation state is +3. It is very much harder to oxidize Am(III) to Am(IV) than it is to oxidise Pu(III) to Pu(IV).

Currently the solvent extraction chemistry of americium is important as in several areas of the world scientists are working on reducing the medium term radiotoxicity of the waste from the reprocessing of used nuclear fuel.

See liquid-liquid extraction for some examples of the solvent extraction of americium.

Americium, unlike uranium, does not readily form a dioxide americyl core (AmO2). [cite journal | journal = Los Alamos Science | format = Reprinted at fas.org | title = The Chemical Complexities of Plutonium | author = David L. Clark | issue = 26 | year = 2000 | url = http://fas.org/sgp/othergov/doe/lanl/pubs/00818038.pdf] This is because americium is very hard to oxidise above the +3 oxidation state when it is in an aqueous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which tend to be readily mobile with low affinities to soil.